Abstract. The spall strength of polydimethylsiloxane silicone oils has been studied using the planar impact of thin flyers to generate large transient negative pressures near the free surface of target samples. The liquids were contained within sealed capsules in which a 4-µm-thick aluminized Mylar diaphragm formed a free surface at the back of the sample. The liquid targets were impacted by PMMA flyers at velocities ranging from 130 to 700 m/s using a 64-mm-bore gas-gun, thus allowing for large variations in the strain rate and incident shock pressure. The peak tension in the liquid was determined by monitoring the free surface velocity using a photonic Doppler velocimetry (PDV) system. The paper focuses on the study of a system of silicone oils having vastly different viscosities (5 cSt to 1000 cSt), but otherwise similar liquid properties. The effect of viscosity on spall strength is compared to previously published data and models.